Materials for absorbing electromagnetic power and converting the absorbed energy to heat in situ may be used for purposes such as microwave cooking, pipe joining, or cable splicing. Such materials are typically a composite of one or more kinds of dissipative materials in combination with a dielectric material.
In the microwave range (above about 2000 MHz), efficient heat generation may occur by coupling electromagnetic power to the electrical dipoles of the dielectric material, thereby causing the dipoles to resonate. For many applications, however, using electromagnetic power at these high frequencies may be impractical due to the need to contain radiation for safety reasons.
At lower electromagnetic power frequencies, electrical dipole coupling is not an efficient means of heat generation. Alternatively, heating may be accomplished by methods such as magnetic induction and magnetic resonance. In the case of magnetic resonance heating, radio frequency (RF) power in the form of an oscillating magnetic field may be coupled to perpendicularly oriented magnetic spins in a magnetic material contained in an absorbing composite. Ferrites have been used as the magnetic material in such RF-power-absorbing composites, despite having some disadvantages. For example, the maximum permeability of ferrites is limited relative to that of metal alloys. Furthermore, it is difficult to form ferrites into particles having a thin needle- or plate-like shape so as to allow efficient penetration of the magnetic field into the particles. Ferrite powders instead comprise particles which are roughly spherical in shape. As a result, the magnetic field tends to become depolarized in the ferrite particle, thereby limiting the bulk permeability of the absorbing material and the overall energy-to-heat conversion efficiency.